The present invention relates generally to power factor correction (“PFC”) circuits. More particularly, the present invention relates to totem-pole PFC circuits implementing current sense transformers and shunt circuitry to prevent damage from reverse current flow.
Many conventional PFC circuits for AC-DC or AC-AC power converters are configured with a diode bridge rectifier at an input stage, followed by for example a boost circuit. Efficiency for such configurations can be inherently limited, in that there are two diode voltage drops at the input stage and one at the boost stage. This is problematic for prospective designs, as the efficiency requirements for power converters are constantly being pushed higher and higher.
Totem Pole PFC circuits are known in the art for being able to achieve higher efficiency by eliminating the power loss in the input bridge rectifier. As compared to other bridgeless alternatives, the totem-pole PFC configuration uses the fewest components and has the smallest conduction loss.
An exemplary totem-pole PFC configuration 10 as is known in the art may now be described, further by reference to FIG. 1. In a first (e.g., positive) AC line half-cycle, switching element Q4 is turned on for line rectification purposes via PWM signals (e.g., PWM_L) from an associated PWM controller 11, further wherein the AC source (Vac) is connected to the load (RL) return. Switching element Q2 is controlled via PWM_B signals from controller 11 as an active boost switch during this phase, and switching element Q1 freewheels the inductor current into the output capacitor C1 and load RL. During the freewheeling phase for the switching element Q1, the switching element Q1 may also typically be turned on in complementary fashion with respect to the active switching to minimize conduction losses.
In a second (e.g., negative) AC line half cycle, switching element Q3 is controlled via PWM signals (e.g., PWM_H) from the PWM controller 11 to connect the AC source to the output DC bus (Vbulk) node. In this phase, the switching element Q1 is the active boost switch and switching element Q2 freewheels the inductor current.
Typically, the controller 11 is configured to detect each AC zero crossing and alternate operating modes accordingly. The controller therefore needs to sense at least the input voltage.
Current sense transformers are known in the art for sampling current, but in a totem-pole configuration it may be understood that negative current will pass through the current sense transformer in freewheeling phases (as further described below), creating undesired signals which may further cause abnormal operation in the control circuit 11.
Therefore, Hall sensors have typically been applied in totem-pole PFC topologies to sample current, but they are relatively bulky and costly for most practical power converter applications.